It's about +0.05 to +0.07 for my CPU for each 100 MHz around the voltages that are interesting. It seem to need a bit more around 1.4V. So if your CPU behaves a bit like mine, if you are sure the 1.29V are the lowest stable for 4.9, I'd guess 1.35V or 1.36V for 5 GHz. That should still be a pretty conservative voltage. Using offset overclocking, any spikes in voltage would still be below 1.4V, I'd think.